Note: this guide is currently under construction.
Suthorn Ruins Miniboss - Shadow Link
We successfully found Link, but he too has turned evil after being pulled into the Still World. The key here is to distract Link and force him to look in one direction while attacking him from behind. You can either distract him yourself and let your echoes attack from behind, or you can have them distract and throw things at him from behind.
The Peahat echo is a great option as it will naturally float above and behind to attack. If you don't have the Peahat, a combination of a Spear Moblin and Rope can also work. After taking several hits, Link will transition to a second phase where he can use a spin attack. This can take out echoes more quickly but the strategy to defeat him in this phase is the same.
After you defeat Link, he will leave behind the mysterious sword. You can now press up on the d-pad to enter swordfighter form. This lets you temporarily become an echo of Link, using his sword, shield, and higher jump. Being in this form depletes an energy gauge that you can refill with blue blobs of energy that you can now find floating around.
Suthorn Ruins Boss - Seismic Talus
Enter through the boss door to find yourself in an arena with the first boss of the game: Stone Talus. This guy is made of rock so most attacks will not be effective against him. The key is to bind to the glowing purple stone and pull it away from the rest of his body. After you do, you'll have a brief window to damage the stone. You can summon an echo like a Peahat to damage it or use swordfighter form. Or both. The glowing stone can move from below him to next to his arms, and you'll have to wait for him to lower his arm or put his hands out for you to be able to reach it.
After a few cycles of this the Stone Talus will turn a darker color and enter phase 2. His attacks are a little faster now. He also will keep his purple stone in his head, so the only time you'll be able to bind to it is when he lowers his head after slamming his hands down. Make sure to bind to it quickly and start pulling directly backwards, as it's stuck in his head pretty well. Just continue to slash at it once it's loosened until the Stone Talus is defeated.
After the boss battle you'll get a full Heart Container to increase your health. You will also have freed all of Tri's friends that were stuck here, allowing them to repair the rift. Tri will also increase to Lv. 2, allowing you to summon more echoes and use swordfighter form for longer. You'll also get 5 unusual crystals.
Gerudo Desert Miniboss - Lanmola
To the northwest of the Oasis in Gerudo Desert you'll find a Lanmola in the sandstorm. It's a sand worm that fights in a deep pit of sand. There are a couple platforms that you can safely stand on during the fight without sinking.
To defeat the Lanmola you'll need to wait for it to bury itself in the ground until only it's tail is sticking out and then bind yourself to the tail. You can then pull it out and expose its soft underbelly. You can then summon echoes and/or use swordfighter form to deal a lot of damage. There is no echo for defeating it but you will get many rupees and the sandstorm in the area will be cleared.
Gerudo Sanctum Miniboss - Shadow Link (Bow)
Just like in Suthorn Ruins, the miniboss of this dungeon is Shadow Link. This time around, he'll have a bow at his disposal. He can out manuever most of your echoes, so dealing damage in swordfighter form is your best bet. After dealing a few hits of damage, he will split into three.
The strategy for the second phase is the same as the first, you'll just have some more arrows to dodge. Each Shadow Link copy will only take two hits from your sword to defeat, it's just a matter of catching them. After he's been defeated you can pick up the Bow of Might. This is another weapon that you can use in swordfighter form.
Gerudo Sanctum Boss - Mogryph
Mogryph is a giant mole monster who will burrow in the sand before exposing himself to attack. He attacks by swiping in the air, which will also cause a ripple in the sand that can damage you. You can avoid this by not standing directly in front of him. Spawn a couple Sand Piranha echoes as they can deal damage even while he is burrowed. Otherwise just wait until he exposes himself and use swordfighter form. You can also trick him into charging into a statue, which will stun him for awhile.
In phase two of the fight, Mogryph will create quicksand in parts of the arena. He'll also grow wings and begin to fly around, spawning large tornadoes to attack. You'll just want to avoid his attacks when he is flying around and wait for him to eventually come back down. When he does he'll do a charge attack after landing once. Place a statue in front of him when he charges to stun him and then wail on him in swordfighter form.
After repeating this process of stunning him by forcing him to run into statues a few times, Mogryph will be defeated. Defeating him will complete the dungeon and free Tri's friends, allowing him to power up. You will also get 5 Might Crystals and a Heart Container as a reward.